Respect initial key convention (_ delimited)

swh/model/identifiers.py

@@ -265,8 +265,8 @@ def revision_identifier(revision):
     ])
 
     metadata = revision.get('metadata', {})
-    if 'extra-headers' in metadata:
-        headers = metadata['extra-headers']
+    if 'extra_headers' in metadata:
+        headers = metadata['extra_headers']
         keys = list(headers.keys())
         keys.sort()
         for header_key in keys:

swh/model/tests/test_identifiers.py

@@ -289,7 +289,7 @@ class RevisionIdentifier(unittest.TestCase):
                                                 tzinfo=linus_tz),
             'message': b'Linux 4.2-rc2\n',
             'metadata': {
-                'extra-headers': {
+                'extra_headers': {
                     'svn-revision': 10,
                     'svn-repo-uuid': '046f1af7-66c2-d61b-5410-ce57b7db7bff',
                 }